Proton conductive polymers are attractive materials for use in applications such as polymer electrolyte fuel cells (PEFCs) for power generation. However, the types of proton conductive polymers which may be used as membranes in PEFCs is limited by demanding membrane requirements such as good chemical and mechanical stability, high ionic conductivity, and low reactant permeability (i.e. hydrogen or methanol, and oxygen).
The art has focused on membranes made from sulfonic acid functionalized polymers, in particular, membranes such as Nafion™ formed from perfluorosulfonic acid functionalized polymers.
Attractive alternatives to sulfonic acid containing materials for use in membranes include sulfonimide groups. The high acid strength of sulfonimide acids is well known. DesMarteau et al., J. Fluorine Chem. 1995, 72, 203-208 and U.S. Pat. No. 5,463,005, prepared perfluorinated polymeric membranes containing sulfonimide acid groups. DesMarteau et al. also described synthesis of trifluorovinyl aromatic ether monomers functionalized with both pendent sulfonimide groups as well as sulfonimide groups incorporated into the monomer main chain. These monomers undergo thermal cyclopolymerization to yield perfluorocyclobutane aromatic polyethers.
Sulfonimide-functionalized polymers which include aromatic units also have been developed. Feiring et al. synthesized a styrene monomer functionalized by a pendent sulfonimide group. Feiring et al. also homopolymerized and copolymerized the functionalized styrene monomer with a variety of olefinic monomers for potential use as electrolytes in lithium batteries.
Polyphosphazenes are a class of polymers which contain a flexible backbone of —P═N— repeating units and two organic, inorganic, or organometallic groups attached to each phosphorus atom. Polyphosphazenes have a phosphorous-nitrogen sequence with organic substituents on the phosphorous atom as follows:
where Rz and R′(2-z) are the same or different organic substituents and 0<z<2.
These polymers can be prepared by the thermal ring opening polymerization of hexachlorocyclotriphosphazene or by the living cationic polymerization of phosphoranimines to form poly(dichlorophosphazene) which is employed as a reactive macromolecular intermediate. The chlorine atoms in this polymer can be replaced via nucleophillic substitution reactions using, for example, alkoxy, aryloxy or amino reagents to give stable poly(organophosphazene) derivatives.
Incorporation of carboxylic, phosphonic, and sulfonic acids into polyphosphazenes is known. Polyphosphazenes functionalized with phosphonic and sulfonic acid groups have been shown to be promising as fuel cell membrane materials, particularly for use in direct methanol fuel cells (DMFCs). See J. Membrane Science, Vol. 119, pg 155 (1996) and Vol. 154, pg. 175 (1999)).
These functionalized polyphosphazene polymers are obtained by treating poly(aryloxyphosphazenes) with relatively harsh reagents such as SO3 to incorporate the acidic functionality. This method limits the choice of functional side groups and thus the degree of tailorability of the phosphazene polymer. Sodium salts of difunctional reagents such as p-hydroxybenzenesulfonic acid are, in general, not suitable reagents for reaction with unsubstituted or partially substituted poly(dichlorophosphazene) due to the tendency of both of the functional sites of the difunctional reagent to cause polymer crosslinks and insoluble products.
A need therefore exists for having the acid functionality incorporated into a side group which then can be reacted with the polyphosphazene or partially substituted derivatives of polyphosphazene.